1. Field of the Invention
This invention relates to the field of voltage surge protection and more particularly to the protection of equipment from large voltage surges and voltage transients such as result from lightning discharges and inductive load switching.
2. Background Information
Voltage surge and voltage transient suppressors are commonly used between a power source and its electrical load. Such suppressors protect the equipment from surges and transients or spikes as may occur on the power line due to switching of inductive loads on the power line or lightning strikes on the power line. In addition, surge suppressors prevent switching transients generated within a load from being reflected back into the power source and to other equipment.
For certain applications, it is necessary that the surge suppressor meet the following characteristics. Spike voltages of amplitudes up to and including a 2500 volt peak as specified in MIL-STD-1399(NAVY), Section 300A, and the standard 1.2.times.50 .mu.s, 6000 V and 8.times.20 .mu.s, 3 kA voltage and current impulse waves respectively, specified in IEEE Standard 587-1980, must be attenuated to a level less than two times the peak voltage of the nominal system voltage. The suppressor must be effective in the frequency range of 2 kHz to 500 kHz. A series type suppressor must not cause more than 0.25% voltage drop at rated load and nominal operating line frequency. If more than one suppressor is used in a series arrangement, the total voltage drop of all units in series must be limited to 0.25% of the line voltage. A series type suppressor must be able to withstand repetitive inrush currents which, for example, in motor circuits can be six times the rated full load current. A shunt type suppressor must be capable of operating at rms voltage levels up to 121% of the nominal line voltage of the system being protected. The suppressor must be able to dissipate the energy contained in the spike as limited by the impedance of the source. The leakage or standby current drawn by the suppressor should be limited to 1% of the rated line current. The requirements of attenuating the spike voltage to a level less than two times the peak voltage of the nominal system voltage, and limiting the voltage drop across the suppressor to 0.25% of the line voltage, are particularly difficult to meet simultaneously.
Several types of devices useful as surge suppressors are known in the prior art. These include gas tubes, silicon avalanche suppressors, capacitors, and metal oxide varistors (MOVs).
A gas tube is basically a spark gap with the electrodes hermetically sealed in a gas-filled ceramic enclosure to lower the breakover (or breakdown) voltage. This type of device is small and inexpensive and has the capability of withstanding pulse currents up to 20000 amperes. When the device breaks over, the typical arc voltage ranges from 10 to 30 volts. However, the breakdown voltage of a spark gap device varies, for at a fixed set of conditions, the breakdown voltage is dependent on the rise time of the applied surge. For example, the typical sparkover voltage for a presently available gas tube rated for a 460 V.sub.ac application ranges from 1100 volts for a 100 volt per microsecond surge rise time to 1500 volts for a 10 kilovolt per microsecond surge rise time. Note that these are typical breakover voltages which are subject to additional variations at distinct surge rise times. As a result, depending on the applied transient, several microseconds may elapse before a typical gas tube arcs over, leaving the leading portion of the surge intact to be passed on to the equipment operating on the power line. Although the gas tube diverts the majority of the surge current when it breaks over, the leading portion of the surge, frequently called a surge remnant, can contain a considerable amount of energy and have a high voltage amplitude. To clip the surge remnant, a common practice is to insert an L-section suppression circuit in the line following the gas tube. This circuit consists of a series impedance and a voltage clamping device, such as a MOV or a silicon avalanche suppressor, connected across the power line. The series impedance is connected between the gas tube and the clamp and can simply be a resistor or an inductor, or both; a resistor being suitable only for low voltage, low current applications. The impedance must be high enough in value to guarantee gas tube breakover so that the clamp only clips and diverts the energy in the remnant, not the energy in the entire surge. A major problem associated with gas tubes is "follow-on" current, the current from the power source which continues flowing through the gas tube after the surge current terminates. In ac circuits, the follow-on current clears when the line current goes through zero but the gas tube could be re-ignited on the next cycle. A typical gas tube is rated to handle a 60 Hz, one-half cycle peak current of only 20 amperes, hence, if the power source can deliver much higher currents, i.e., a 460 V.sub.ac power line, the gas tube could be destroyed, particularly if it breaks over at the beginning of a cycle. In dc applications, a separate means for extinguishing the arc must be included in the circuit. Frequently, the follow-on current is limited to a safe value by connecting a low value resistor or a clamp such as a MOV in series with the gas tube. This technique, however, can significantly raise the clamping voltage if the surge current level is high.
Silicon avalanche suppressors are essentially large junction zener diodes specifically designed for transient protection, functioning as a clamp and providing suppression in just a few nanoseconds. Presently, the major limitation of this device is its low energy dissipation capability as compared with gas tubes and MOV's.
A capacitor placed across the power line is a simple form of surge filter. The impedance of the capacitor forms a voltage divider with the surge source impedance resulting in the attenuation of transients at high frequencies, the higher the capacitance value, the greater the attenuation. Frequently, an inductor is placed in series in the line before the capacitor to form an L-section low pass filter which is an effective transient suppressor. A bi-directional transient suppressor is formed by a T-section low-pass filter which has an inductor in line on either side of a shunt capacitor. This simple approach may have undesirable side effects such as: unwanted resonances with the inductive components located in the circuit; high in-rush currents during turn-on and switching; excessive reactive load on the power system; high leakage current, especially in 400 Hz applications when the capacitance value is high; and, high voltage drop across the inductors. To limit the standby current, an inductor is sometimes connected in parallel with the filter capacitor to form a tank circuit tuned to the power line frequency. Although this allows high values of 'shunt capacitors to be used which provides greater attenuation of transients, a circulating current flows in the tank circuit continuously. Depending on the component values selected, this circulating current may be quite high, resulting in substantial heating of these circuit elements. Usually, such tank circuit components are large and heavy. Also, the problems applicable to low pass filters described above are equally applicable to tank circuits.
Metal Oxide Varistors (MOVs) are devices which clamp and are usually connected directly across a power line. The device does not clamp until a voltage transient (spike) occurs which exceeds the line voltage by a sufficient amount. As the voltage transient rises, the MOV nonlinear impedance results in a spike current through the device which rises faster than the voltage across it. This produces the clamping action of the device. The clamping voltage depends on the line impedance and the impedance of the voltage spike source. If the spike source and line impedance are low, the spike current through the MOV is high, and hence the clamping voltage is high. If the spike source and line impedances are high, both the spike current through the MOV and the clamping voltage are low. When the spike source impedance is very low, several thousand amperes can flow through the MOV. Although MOVs can handle currents of this magnitude, they can do so only for a limited number of times before the device fails. To reduce the surge current through the MOV and thereby the clamping voltage, an inductor is connected in series to form an L-section as with a capacitor. By adding an inductor in the line on either side of the MOV, a bi-directional T-section transient suppressor is formed. As the series inductance value is increased, the surge current through the MOV is decreased. This results in a lower clamping voltage, but at the cost of high line voltage drop across the inductors. Hence, with this approach, the ability to limit a spike voltage amplitude of up to 6000 V peak to a level of less than two times the peak voltage of the nominal system voltage, and the ability to limit the voltage drop to less than 0.25% at rated load, and nominal operating line frequency cannot be met simultaneously.